The present invention relates generally to navigation systems. More particularly, the present invention is related to a system and method of operating a global positioning system and related architecture.
The current global positioning system (GPS) provides timing and navigation for a wide range of applications, such as intelligent transportation systems, telecommunications, delivery of military munitions, and power control grids. The applications may be military or civilian related. Civilian applications include commercial and noncommercial applications.
The GPS is designed to provide three-dimensional navigation anywhere in the world, at any time, and under all weather conditions. The GPS generates a pair of timing signals that are broadcasted on two frequencies of the L-band. The two frequencies are designated as L1 and L2. Time, range, and position information of each satellite can be obtained from the timing signal. A GPS receiver can use time of arrival information to determine range information. The receiver, by receiving multiple timing signals from multiple satellites, can also determine position information. Each satellite transmits a unique code, which enables the satellites within the GPS system to use the same L1 and L2 frequencies. The timing signal on the L1 frequency is broadcasted having a short unencrypted code that is used for both military and civilian applications. The coded timing signal on the L2 frequency is broadcasted having a longer encrypted code that is only used for authorized users of the United States military and its allies.
The use of the two frequencies allows the military GPS receivers to account for some atmospheric effects on the ranging signals. Thus, the military operated GPS receivers are typically more accurate than the civilian receivers. The military receivers are accurate to approximately within ten meters (see the Precise Positioning Service Performance Standard for military specification standards) as opposed to the civilian receivers, which are accurate to approximately within tens of meters (see the Standard Positioning Service Performance Standard for civilian specification standards).
The current GPS is not designed for civilian aviation applications that require integrity or guaranteed position accuracy, and is therefore limited for aviation use. Civilian aviation requirements, which are meant to assure accuracy of the GPS received signals and prevent collisions or injuries to vehicles and occupants therein, are more stringent than existing GPS operating requirements for such applications. Civilian requirements include an assured accuracy requirement that is on the order of approximately ten meters in the vertical direction. Civilian requirements also include a “time to alert” requirement that refers to a maximum allowable amount of time to notify a pilot when a navigation system malfunction or inaccuracy exists within the GPS signals. When a malfunction that is undesirable exists within the GPS, the GPS navigation solution or resulting data is no longer safe for use.
There currently exists several regional satellite health monitoring systems including the European Geostationary Navigation Overlay Service (EGNOS), the MTSAT Satellite Augmentation System (MSAS), and the Wide Area Augmentation System (WAAS). These systems monitor integrity of the GPS signals and determine errors associated with positions and clocks of satellites, and atmospheric attenuation. The error information along with other integrity information is transmitted to the geostationary satellites, which is then retransmitted such that an aircraft may determine a current GPS status and errors associated with the received GPS signals. These monitoring systems enable trusted, but limited vertical navigation over selected service volumes, such as the continental United States for the WAAS, western Europe for the EGNOS, and mainland Japan for the MSAS.
The current GPS requirement for response time for a stand-alone GPS without additional regional monitoring is approximately six hours. Actual response time varies from approximately one half an hour to four hours. This is unacceptable for civilian applications that require received and used navigation values to be reliable. Such a long time delay to warn can result in a collision and injury to an aircraft and occupants therein.
Although the GPS-enabled avionics is relatively inexpensive to deploy, accuracy and time to alert deficiencies of the GPS prevent it from being relied upon during the landing phase of civilian flight. Currently, in low visibility conditions, a civilian aircraft is capable of landing through use of an instrument landing system (ILS). The pilot of the aircraft monitors an instrument that senses a radar signal from the ground and in response thereto performs a landing. However, the ILS is expensive and is only available at a limited number of airports, which are typically large commercial airports, such as international airports. It is unsafe to land an aircraft in a low visibility area without use of an ILS system and is generally not permitted by regional civil aviation authorities.
Thus, there exists a need for an improved navigation system that is accurate, provides integrity information quickly, is global, and is inexpensive such that it may be used and relied upon for civilian aviation applications.